1. Field of the Invention
The present invention relates to an image quality stabilization method for an image forming apparatus.
2. Description of the Related Art
In recent years, with the widespread use of electrophotographic type image forming apparatuses and inkjet type image forming apparatuses, it may be desired by the market that an image forming apparatus is capable of forming an image of a high image quality. The image quality may be caused by density unevenness (a phenomenon so-called “banding”) of a sheet in its conveyance direction (in a sub scanning direction).
In order to suppress degradation of image quality caused by density unevenness, Japanese Patent Application Laid-Open No. 2007-108246 discusses a method for suppressing density unevenness occurring in the sub scanning direction. The method discussed in Japanese Patent Application Laid-Open No. 2007-108246 measures density unevenness in the sub scanning direction, which may occur according to an outer diameter period of a photosensitive drum, in advance in relation to the phase of the photosensitive drum. In addition, this conventional method stores a result of the measurement in a storage unit as a density pattern information table. Furthermore, the conventional method reads information about the density unevenness, which is measured according to the phase of the photosensitive drum during image formation processing, from the density pattern information table. Moreover, the conventional method corrects the density unevenness that may occur according to the outer diameter rotational period of the photosensitive drum by using the information about the density unevenness.
After examining an image quality that can be achieved according to the above-described conventional method, it was found by the applicant of the present invention that unevenness of rotation of a motor that drives a photosensitive drum (periodical variation of the rotational speed) should be considered as a cause of density unevenness occurring in the sub scanning direction. To paraphrase this, when a motor is driven and rotated, rotational unevenness of the motor may arise due to the configuration of the motor itself, i.e., the number of magnetized poles thereof. Furthermore, the motor rotation unevenness may lead to density unevenness, which may cause image degradation.
On the other hand, the above-described method discussed in Japanese Patent Application Laid-Open No. 2007-108246 can correct density unevenness that may occur according to an outer diameter period of the photosensitive drum but cannot correct density unevenness that may occur in a short period, which may be caused by rotational unevenness of a motor. More specifically, if the manufacture accuracy of mechanical parts related to a motor is low due to reduction of costs of manufacture of the motor, the density unevenness occurring in a short rotational period of a motor may increase. In other words, in this case, in order to achieve a high quality image, effectively reducing density unevenness that may arise due to rotational unevenness of a motor is to be performed.